1. Field of the Invention
The present invention relates to a gradient magnetic field coil unit, which has a passive shimming mechanism for correcting the homogeneity of a magnetic field, a gantry for MRI (Magnetic Resonance Imaging) apparatus, and an MRI apparatus.
2. Description of the Related Art
MRI apparatuses have a static magnetic field magnet which generates an extremely strong static magnetic field and is formed in a substantially cylindrical shape. In the static magnetic field generated by the static magnetic field magnet, the strength of several kilo gauss to 10 kilo gauss (1 telsa) is typically required. In addition to the strength, a spatial homogeneity is required in the static magnetic field. As for the spatial homogeneity, the homogeneity of less than several dozen ppm is needed. A spatial region in which a homogeneous magnetic field is required is a spherical region with a diameter of about 50 cm.
The magnetic field is distorted by the effect of a magnetic body. As for a method for correcting the heterogeneity of the static magnetic field, to which the phenomenon is applied, there is provided a method in which a magnetic body such as an iron piece (iron shim) is disposed in a proper position of a gradient magnetic field coil unit. The method is referred to as passive shim. Further, an operation for increasing the homogeneity of a magnetic field like a passive shim or the like is referred to as shimming.
Inside the static magnetic field magnet, a gradient magnetic field coil unit for generating a gradient magnetic field is attached. The gradient magnetic field coil unit includes a main coil layer having a main coil for generating a gradient magnetic field and a shield coil layer having a shield coil for generating a magnetic field which shields a leakage magnetic field from the main coil. The coupling of the gradient magnetic field from the main coil and the magnetic field from the shield coil reduces the magnetic field strength within an imaging region. Therefore, it is preferable to separate the main coil and the shield coil from each other, if possible. Accordingly, a shimming layer which houses an iron shim for correcting the heterogeneity of a static magnetic field is provided between the main coil layer and the shield coil layer.
Recently, to enhance the image quality of an MR image, a demand for increasing the strength of a gradient magnetic field is growing. To increase the gradient magnetic field strength, a high current needs to be supplied to the gradient magnetic field coil unit. As a high current is supplied, the heating value of the main coil and the shield coil increases. Therefore, the temperature of the iron shim installed in the gradient magnetic field coil unit increases. As the temperature of the iron shim increases, the homogeneity of the static magnetic field is varied. Accordingly, a main coil cooling layer having a cooling tube for cooling the main coil is provided between the main coil layer and the shimming layer, and a shield coil cooling layer having a cooling tube for cooling the shield coil is provided between the shield coil layer and the shimming layer. These cooling tubes are attached in a spiral shape along the substantially cylindrical shape of the gradient magnetic field coil unit (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-311957).
Meanwhile, a demand for widening a bore diameter is growing, in order to improve the interior comfort of a patient. To widen a bore diameter without changing the inner diameter of the static magnetic field magnet, the inner diameter of the gradient magnetic field coil unit needs to be widened. Therefore, it is difficult to secure a sufficient space in the main coil cooling layer, the shimming layer, and the shield coil cooling layer, which are present between the main coil layer and the shield coil layer.